(D)Atmospheric pressure can support a column of water up to 10 meters high. But plantscan move water much higher, the sequoia tree can pump water to its very top, more than 100meters above the ground. Until the end of the nineteenth century, the movement of water intrees and other tall plants was a mystery. Some botanists hypothesized that the living cells ofplants acted as pumps, but many experiments demonstrated that the stems of plants in whichall the cells are killed can still move water to appreciable heights. Other explanations for themovement of water in plants have been based on root pressure, a push on the water from theroots at the bottom of the plant. But root pressure is not nearly great enough to push water tothe tops of tall trees, Furthermore, the conifers, which are among the tallest trees haveunusually low root pressures.   If water is not pumped to the top of a tall tree, and if it is not pushed, to the top of a talltree, then we may ask. How does it get there? According to the currently acceptedcohesion-tension theory, water is pulled there. The pull on a rising column of water in a plantresults from the evaporation of water at the top of the plant. As water is lost from the surfaceof the leaves, a negative pressure or tension is created. The evaporated water is replaced bywater moving from inside the plant in unbroken columns that extend from the top of a plantto its roots. The same forces that create surface tension in any sample of water areresponsible for the maintenance of these unbroken columns of water. When water is confinedin tubes of very small bore, the forces of cohesion ( the attraction between water molecules)are so great that the strength of a column of water compares with the strength of a steel wireof the same diameter. This cohesive strength permits columns of water to be pulled to greatheights without being broken.
77. The passage answers which of the following questions?
(A) What is the effect of atmospheric pressure on foliage?
(B) When do dead cells harm plant growth?
(C) How does water get to the tops of trees?
(D) Why is root pressure weak?